Researchers at Rice University were able to demonstrate photon upconversion from 2.4 to 2.8 eV using GaN/InGaN quantum wells enhanced with gold and silver nanoparticles. They were also able to show that the process scales linearly with illumination power and that it enables both geometry- and polarization-based tunability.

Professor Gururaj Naik is developing technology to upconvert light by using lasers to power devices that combine plasmonic metals and semiconducting quantum wells. Courtesy of Tommy LaVergne/Rice University.
“Upconverters based on lanthanides and organic molecules emit and absorb light at set frequencies because they’re fixed by atomic or molecular energy levels,” said professor Gururaj Naik. “We can design quantum wells and tune their bandgaps to emit photons in the frequency range we want and similarly design metal nanostructures to absorb at different frequencies. That means we can design absorption and emission almost independently, which was not possible before.”.

In experiments, when pylons measuring about 100 nm across were excited by a specific wavelength of light, gold nanoparticles on the tips of the pylons converted the light energy into plasmons. The short-lived plasmons decayed, giving up their energy by emitting a photon, or producing heat by transferring their energy to a hot electron.

The pylons were designed using alternate layers of gallium nitride and indium gallium nitride topped with a thin layer of gold and surrounded by silver.

Instead of letting the hot carriers slip away, researchers directed both hot electrons and hot holes toward the GaN and InGaN bases, which served as electron-trapping quantum wells. These wells have an inherent bandgap that sequesters electrons and holes until they recombine at sufficient energy to release photons at a higher frequency.

Efficient upconversion of light could let solar cells turn otherwise-wasted IR sunlight into electricity or help light-activated nanoparticles treat diseased cells, said Naik, whose work was inspired by the groundbreaking research of professors Naomi Halas and Peter Nordlander at Rice’s Laboratory for Nanophotonics that showed that exciting plasmonic materials also excited “hot carriers” — electrons and holes — within.

“Plasmonics is really great at squeezing light on the nanoscale,” said Naik. “But that always comes at the cost of something. Halas and Nordlander showed you can extract the optical losses in the form of electricity. My idea was to put them back to optical form.”

According to Naik, “The next step is to make standalone particles by coating quantum dots with metal at just the right size and shape. These show promise as medical contrast agents or drug-delivery vehicles.

“Infrared light penetrates deeper into tissues, and blue light can cause the reactions necessary for the delivery of medicine,” he said. “People use upconverters with drugs, deliver them to the desired part of the body, and shine infrared light from the outside to deliver and activate the drug.”

The particles could also be used to make invisible ink.

“You can write with an upconverter and nobody would know until you shine high-intensity infrared on it and it upconverts to visible light,” said Naik.

A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.